In another question I asked a question about this sentence

For example these compounds are appropriate replacement for volatile organic compounds (VOCs) due to their low vapor pressure.

As a side node, the answer corrected it to:

For example these compounds are an appropriate replacement for volatile organic compounds (VOCs) due to their low vapor pressure.

But I asked could it be:

For example these compounds are appropriate replacements for volatile organic compounds (VOCs) due to their low vapor pressure.

Because the subject is plural?

It seems both sentence are possible, right? if yes why?

I felt after are I need a plural case, isn't it always right?

In another question I asked a question about this sentence

For example these compounds are appropriate replacement for volatile organic compounds (VOCs) due to their low vapor pressure.

As a side node, the answer corrected it to:

For example these compounds are an appropriate replacement for volatile organic compounds (VOCs) due to their low vapor pressure.

But I asked could it be:

For example these compounds are appropriate replacements for volatile organic compounds (VOCs) due to their low vapor pressure.

Because the subject is plural?

It seems both sentence are possible, right? if yes why?

I felt after are I need a plural case, isn't it always right?

In another question I asked a question about this sentence

For example these compounds are appropriate replacement for volatile organic compounds (VOCs) due to their low vapor pressure.

As a side node, the answer corrected it to:

For example these compounds are an appropriate replacement for volatile organic compounds (VOCs) due to their low vapor pressure.

But I asked could it be:

For example these compounds are appropriate replacements for volatile organic compounds (VOCs) due to their low vapor pressure.

Because the subject is plural?

It seems both sentence are possible, right? if yes why?

In another question I asked a question about this sentence

For example these compounds are appropriate replacement for volatile organic compounds (VOCs) due to their low vapor pressure.

As a side node, the answer corrected it to:

For example these compounds are an appropriate replacement for volatile organic compounds (VOCs) due to their low vapor pressure.

But I asked could it be:

For example these compounds are appropriate replacements for volatile organic compounds (VOCs) due to their low vapor pressure.

Because the subject is plural?

It seems both sentence are possible, right? if yes why?

I felt after are I need a plural case, isn't it always right?